A known load drive control device controls driving of an inductive load by generating pulse signals with a predetermined duty ratio at a predetermined drive frequency.
In the inductive load driven at the predetermined drive frequency, resonance is generated when the drive frequency coincides substantially with a natural vibration frequency of the inductive load, or is substantially equal to the integral multiple of the natural vibration frequency of the inductive load. In order to prevent the resonance, it is necessary to determine a drive frequency based on the natural vibration frequency. However, when the natural vibration frequency changes due to an external factor, it is difficult to store the drive frequency in advance so that no resonance is generated.
When an inductive load is, for example, a solenoid of a hydraulic valve for controlling oil pressure applied to an automatic transmission, it is preferable to prevent the resonance because the resonance causes, for example, oil pressure pulsation of a valve body. However, because the viscosity of oil varies with, for example, temperature, it is difficult to store the drive frequency in advance so that no resonance is generated.
In this relation, JP-A-2003-167604 describes a control device of hydraulic equipment. In the control device, a stroke sensor detects the position of a spool linked to a plunger of a solenoid serving as an inductive load.
The stroke sensor makes it possible to switch the drive frequency based on a detected amplitude.
As described above, a known load drive control device may switch the drive frequency based on the amplitude detected by the stroke sensor. Consequently, the load drive control device may detect a resonant condition from the amplitude and switch the frequency to a drive frequency that causes no resonance generation.
The above load drive control device requires the stroke sensor in order to suppress the resonance. The stroke sensor is very expensive, and a man-hour for mounting the stroke sensor also arises.